In previous applications, systems and apparatus for creating modular subfloors have been disclosed, such as U.S. Pat. Nos. 6,306,477, 6,298,624, 6,395,362 and U.S. application Ser. Nos. 09/617,229, 10/004,834, 10/013,688 all of which are incorporated herein by reference. These applications generally describe plastic plates with a field of hooks on one side. In some of these applications, it has been disclosed that it is often preferable to have a resilient layer attached to the side of the plates on the opposite side from the field of hooks.
Methods of attaching, for example, a resilient layer made of polyethylene foam to a plate made of polypropylene are not well known. For example, they may be attached by gluing or by casting the foam to the plate. Alternatively, a soldering-type gun or hot plate could be used, but undesirable destruction of the foam material could occur. The heat of the gun or hot plate could also cause the polypropylene plate to warp. It has been suggested that bonding a foam resilient layer made of a polyethylene to a polypropylene plate by ultrasonic welding would not provide a sufficient bond between the resilient layer and the plate to enable the resilient layer and plate to be used in a sub-floor.
In order to weld or otherwise bond polyethylene foam and a polypropylene plate, a welding of polypropylene and polyethylene is required. Attempting to bond these two different materials can lead to low quality welds. The first reason for this behaviour is that, because of the different chemical structures of the two materials, the materials tend not to chemically bond to each other. The second problem is related to the different thermophysical behaviour of the materials. Polyethylene melts at lower temperatures (having a melting point of about 120° C.) than polypropylene (having a melting point about 165° C.). Accordingly, when the polypropylene begins to melt, the polyethylene is typically already in a liquid state. The result is generally low welding joint strength.
Accordingly, there is a need for alternative methods for welding two different materials such as polyethylene and polypropylene for use on a flooring structure.